I have been using the VMM for a bit now with over 200 items in it. There were some growing pains and early issues and most seem to have been worked out. When I have a product ready, I load up the LL viewer, add the items to the VMM folder, log out and go back to Firestorm. Everything else is good from there on out.
Only downside is that I can't wear any of my clothes from the Marketplace folder as that will cause listing and perhaps delivery issues. So if I want to wear something of my own, I must make a copy of the MP folder and place it elsewhere in my inventory. If I did that for everything, then it would effectively double the size of my inventory.
That, however, raises a question. When they migrate my store, will I automatically get 800 new folders in my inventory with the contents of the MP listings? I hate to think how big my inventory will be now...